Aqueous surface-coating agent for paper and paperboard

ABSTRACT

The present invention relates to an aqueous surface coating composition for paper and board with a solids content of 1 to 55 wt % comprising
     (A) 0 to 20 parts by weight of starch (solids),   (B) 0.01 to 20 parts by weight of a zirconium carbonate compound,   (C) 0.01 to 40 parts by weight of water-soluble synthetic polymer (solids) containing in copolymerized form one or more monomers having monoethylenic double bonds,
 
and also to a method for producing paper and board using the aqueous surface coating composition, and to corrugated boards produced from this paper.

The present invention relates to an aqueous surface coating composition for paper and board with a solids content of 1 to 55 wt % comprising

-   (A) 0 to 20 parts by weight of starch (solids), -   (B) 0.01 to 20 parts by weight of a zirconium carbonate compound, -   (C) 0.01 to 40 parts by weight of water-soluble synthetic polymer     (solids) containing in copolymerized form one or more monomers     having monoethylenic double bonds.

The present invention further relates to a method for producing paper and board using the aqueous surface coating composition, and to corrugated boards produced from this paper.

An important requirement in the packaging paper segment is the strength of the paper, since an important basis for such paper are recycled fibers, which lose length as a result of the recycling and hence lead to a successive decrease in the strength of the paper. To enhance the qualities of the paper, the paper stock is often admixed with assistants such as wet and dry strength agents, such as cationic and anionic polyacrylamide or polyvinylamine, retention agents, and sizing agents. The effect of cationic strengtheners in the paper stock, however, is partly undone by anionic compounds from the recycling process. Attempts are therefore made to obtain additional strength through the adding of assistants to the paper sheet, in the size press, for example.

In surface sizing it is the paper sheet that is coated. Surface sizing agents employed are often gelatin or derivatives of starch. Starch of this kind, added as a surface sizing agent, likewise has a strengthening effect. But it is not possible to increase strength by starch ad infinitum.

WO 2004/027149 teaches accordingly the strengthening of the paper sheet by crosslinking of zinc borate and starch applied to the paper sheet in the size press.

Moreover, EP 2 432 934 teaches improving the strength of paper and card by spraying zirconium carbonate onto a paper sheet made from paper fibers and polyvinyl alcohol fibers, and then drying the coated sheet.

A disadvantage of all of these crosslinking agents is that the process of crosslinking begins in the mixture itself and produces only a limited strengthening effect. An object of the present invention was to provide a surface coating composition which crosslinks only on the fiber and produces high strength in the paper.

Found accordingly has been the aforementioned surface coating composition, and also a method for producing paper and board using the aqueous surface coating composition, and additionally correlated boards produced from this paper.

Paper stock (also known as pulp furnish) refers hereinafter to a mixture of water and fibrous material and, depending on the stage in the papermaking process, may further comprise filler and also paper auxiliaries. Dry paper stock is to be understood as meaning the overall paper stock composed of fibrous material and also, optionally, filler and, optionally, paper auxiliaries, without water (paper stock solids).

The term for the shaped body consisting of fibrous material alters with the mass per unit area, also referred to in the jargon as grammage. Hereinafter, paper is to comprehend a mass per unit area of 7 g/m² to 225 g/m², and board a mass per unit area of 225 g/m² and above.

Inventively, the surface coating composition comprises a zirconium carbonate compound. This zirconium carbonate compound is, for example, ammonium zirconium carbonate or potassium zirconium carbonate. These are anionic, inorganic, hydroxylated zirconium compounds, and are available as water-based solutions and are typically employed for paper coatings, colored coating slips, and liquid-ink formulations.

Starch in this context is to be understood as any virgin, modified, or degraded starch. Virgin starches may consist of amylose, amylopectin, or mixtures thereof. Modified starches may comprise oxidized starch, starch esters, or starch ethers.

Types of starch contemplated include virgin starches such as potato starch, wheat starch, corn starch, rice starch, or tapioca starch, preferably potato starch. Chemically modified starches may also be used, such as hydroxyethyl or hydroxypropyl starches, or else starches which contain anionic groups, such as phosphate starch, or else cationized starches containing quaternary ammonium groups, preference being given to a degree of substitution DS of 0.01 to 0.2. This degree of substitution DS indicates the number of cationic groups present on average in the starch per glucose unit. Particularly preferred are amphoteric starches, which contain not only quaternary ammonium groups but also anionic groups such as carboxylate and/or phosphate groups, and which may optionally also have undergone chemical modification, having for example been hydroxyalkylated or alkyl-esterified. The starches may be used individually or else in any desired mixtures with one another.

Preference is given to the use of digested (degraded) starch. In fully digested starch, the starch grains are completely burst open, with the starch being present in a molecularly disperse form. The average molar masses M_(w) of a degraded starch are situated, for example, in the range from 0.4 million to 8 million daltons, preferably in the range from 0.5 to 3 million daltons, more preferably in the range from 0.6 to 2 million daltons.

Degradation may take place thermally, which normally refers to boiled starch. Degradation may also take place enzymatically. Lastly, degradation may also take place oxidatively. Particular preference is given to using enzymaticaly degraded starch.

The average molar mass M_(w) (determined by GPC) of the water-soluble synthetic polymer is preferably ≤1 million Daltons, for example, 20 000 daltons to 1 million daltons, preferably 35 000 daltons to 1 million daltons. These polymers have example K values (determined by the method of H. Fikentscher in 5% strength aqueous sodium chloride solution at a pH of 7, a polymer concentration of 0.5 wt %, at a temperature of 25° C.) in the range from 20 to 250, preferably 30 to 80.

The water-soluble synthetic polymer preferably contains in copolymerized form one or more monomers selected from acrylamide, vinyl alcohol, vinyl acetate, and an N-vinylcarboxamide of the formula

in which R¹, R²═H or C₁ to C₆ alkyl.

The water-soluble synthetic polymers may be uncharged or charged. The latter carry cationic and/or anionic radicals.

According to one preferred embodiment, the water-soluble synthetic polymer contains in copolymerized form one or more monomers selected from acrylamide and an N-vinylcarboxamide of the formula (I), with the proviso that there is no vinyl alcohol present in copolymerized form.

In connection with the monomers, vinyl alcohol denotes a copolymerized unit [CH₂CHOH] which is customarily obtained through the use as monomer of a vinyl ester, such as vinyl formate or vinyl acetate, for example, and through the subjection of the resulting polymer to hydrolysis, in which the copolymerized vinyl ester monomers are hydrolyzed to [CH₂CHOH] units.

According to one preferred embodiment, water-soluble synthetic polymers that are suitable are the polymers of a vinylcarboxamide of the formula (I) above.

According to one embodiment, suitable water-soluble synthetic polymers are polymers comprising vinylamine units. The cationic polymers comprising vinylamine units are water-soluble. The solubility in water under standard conditions (20° C., 1013 mbar) at a pH of 7.0 amounts for example to at least 5 wt %, preferably at least 10 wt %.

On account of their amino group, the cationic polymers comprising vinylamine units are cationic. The charge density of the cationic polymers comprising vinylamine units (without a counterion) amounts to at least 0.1 meq/g and is preferably in the range from 4 to 10 meq/g.

The cationic polymers comprising vinylamine units typically have average molecular weights M_(w) in the range from 10 000 to 10 000 000 daltons, preferably in the range from 15 000 to 5 000 000 daltons, more preferably in the range from 20 000 to 3 000 000 daltons.

Cationic polymers comprising vinylamine units are known: cf. the cited prior-art DE 35 06 832 A1 and DE 10 2004 056 551 A1.

As cationic polymer comprising vinylamine units, preference is given to using, for example, the reaction products obtainable by polymerizing

-   (a0) at least one monomer of the formula

-   -   in which R¹, R²═H or C₁ to C₆ alkyl,

-   (c0) optionally monoethylenically unsaturated monomers which are     different from the monomers (a0), and

-   (d0) optionally compounds which have at least two ethylenically     unsaturated double bonds in the molecule,

followed by partial or complete hydrolysis of the monomer (I) units copolymerized into the polymer, with formation of amino groups,

and/or obtainable by Hofmann degradation of polymers with acrylamide and/or methacrylamide units.

Hydrolyzing the carboxamide radicals of the copolymerized units of the monomers (I) converts the group —NR²—CO—R¹ into the group —NR²—H. Examples of group (a0) monomers are N-vinyl-formamide, N-vinyl-N-methylformamide, N-vinylacetamide, N-vinyl-N-methylacetamide, N-vinyl-N-ethylacetamide, N-vinyl-N-methylpropionamide, and N-vinylpropionamide. The group (a0) monomers may be used alone or in a mixture in the copolymerization with the monomers of the other groups.

For modification, the copolymers may optionally contain group (c0) monomers in copolymerized form, examples being esters of ethylenically unsaturated C₃ to C₅ carboxylic acids such as methyl acrylate, ethyl acrylate, n-butyl acrylate, isobutyl acrylate, isobutyl methacrylate, methyl methacrylate, ethyl methacrylate, and vinyl esters, such as vinyl acetate or vinyl propionate, for example, or other monomers such as N-vinylpyrrolidone, N-vinylimidazole, acrylamide and/or methacrylamide.

Further modification to the copolymers is possible through use in the copolymerization of monomers (d0) which contain at least two double bonds in the molecule, examples being methylenebisacrylamide, glycol diacrylate, glycol dimethacrylate, glycerol triacrylate, triallylamine, pentaerythritol triallyl ether, polyalkylene glycols at least doubly esterified with acrylic acid and/or methacrylic acid, or polyols such as pentaerythritol, sorbitol, or glucose. If at least one group (d) monomer is used in the copolymerization, the amounts employed are up to 2 mol %, as for example 0.001 to 1 mol %.

In a likewise preferred embodiment, use is made as water-soluble synthetic polymer of reaction products obtainable by polymerizing

-   (a) vinyl acetate and -   (a0) at least one monomer of the formula

-   -   in which R¹, R²═H or C₁ to C₆ alkyl,

-   (c0) optionally monoethylenically unsaturated monomers which are     different from the monomers (a0) and vinyl acetate, and

-   (d0) optionally compounds which have at least two ethylenically     unsaturated double bonds in the molecule,

optionally followed by partial or complete hydrolysis of the monomer (I) units copolymerized into the polymer and of the vinyl acetate units.

The water-soluble synthetic polymer in one preferred embodiment carries acid groups, and is therefore preferably an anionic polymer. The anionic charge density of the water-soluble synthetic polymer (without counterion) amounts to at least −0.1 to 10 meq/g and is preferably in the range from −0.1 to −4 meq/g.

The water-soluble synthetic polymer preferably comprises, more particularly consists of, one or more monomers in copolymerized form, selected from

-   (a) one or more monomers selected from acrylamide, vinyl alcohol,     vinyl acetate, and an N-vinylcarboxamide of the formula

-   -   in which R¹, R²═H or C₁ to C₆ alkyl,

-   (b) one or more monoethylenically unsaturated monomers containing     acid groups, and/or alkali metal, alkaline earth metal or ammonium     salts thereof, and

-   (c) optionally one or more monoethylenically unsaturated monomers     which are different from the monomers (a) and (b), and

-   (d) optionally one or more compounds which have at least two     ethylenically unsaturated double bonds in the molecule.

If the monomers (a) and (b) or (a), (b), and (c) are copolymerized in the presence of a compound (d), branched copolymers are the result. The proportions and reaction conditions in this case should be selected so that water-soluble polymers are still obtained. In certain circumstances it may be necessary, for that purpose, to use chain transfer agents. All known such agents may be used, such as thiols, secondary alcohols, sulfites, phosphites, hypophosphites, thioacids, aldehydes, etc. (further details are found for example in EP-A 438 744, page 5, lines 7-12).

The branched copolymers contain in copolymerized form for example

-   (a) 10 to 95 mol % of units of the formula I, -   (b) 5 to 90 mol % of units of a monoethylenically unsaturated     monomer containing acid groups, and/or of the alkali metal, alkaline     earth metal or ammonium salts thereof, -   (c) 0 to 30 mol % of units of at least one monoethylenically     unsaturated monomer which is different from the monomers (a) and     (b), and -   (d) 0 to 2 mol %, preferably 0.001 to 1 mol %, of units of at least     one compound having at least two ethylenically unsaturated double     bonds.

Examples of group (a) monomers are d examples of group (a) monomers are N-vinylformamide, N-vinyl-N-methylformamide, N-vinylacetamide, N-vinyl-N-methylacetamide, N-vinyl-N-ethylacetamide, N-vinyl-N-methylpropionamide, and N-vinylpropionamide. Another suitable monomer (a) is acrylamide.

Group (b) monomers contemplated include, in particular, monoethylenically unsaturated carboxylic acids having 3 to 8 C atoms, and also the water-soluble salts of these carboxylic acids. This group of monomers includes, for example, acrylic acid, methacrylic acid, dimethacrylic acid, ethacrylic acid, maleic acid, fumaric acid, itaconic acid, citraconic acid, methylenemalonic acid, allylacetic acid, vinylacetic acid, and crotonic acid. Other suitable group (b) monomers include monomers containing sulfo groups such as vinylsulfonic acid, acrylamido-2-methylpropanesulfonic acid, and styrenesulfonic acid, and also vinylphosphonic acid. The monomers of this group may be used alone or in a mixture with one another, in partly neutralized or fully neutralized form, in the copolymerization. Neutralization is done using, for example, alkali metal bases or alkaline earth metal bases, ammonia, amines and/or alkanolamines. Examples thereof are aqueous sodium hydroxide solution, aqueous potassium hydroxide solution, sodium carbonate, potassium carbonate, sodium hydrogen carbonate, magnesium oxide, calcium hydroxide, calcium oxide, triethanolamine, ethanolamine, morpholine, diethylenetriamine, or tetraethylenepentamine. The group (b) monomers are used preferably in partly neutralized form in the copolymerization.

For modification, the copolymers may contain in copolymerized form optionally group (c) monomers, examples being esters of ethylenically unsaturated C₃ to C₅ carboxylic acids such as methyl acrylate, ethyl acrylate, n-butyl acrylate, isobutyl acrylate, isobutyl methacrylate, methyl methacrylate, ethyl methacrylate, and vinyl esters, such as vinyl acetate or vinyl propionate, for example, or other monomers such as N-vinylpyrrolidone and N-vinylimidazole.

For modification, the copolymers may contain in copolymerized form optionally group (d) monomers, examples being methylenebisacrylamide, glycol diacrylate, glycol dimethacrylate, glycerol triacrylate, triallylamine, pentaerythritol triallyl ether, polyalkylene glycols at least doubly esterified with acrylic acid and/or methacrylic acid, or polyols such as pentaerythritol, sorbitol, or glucose. If at least one group (d) monomer is used in the copolymerization, the amounts employed are up to 2 mol %, as for example 0.001 to 1 mol %.

The polymerization of the monomers takes place in a known way in the presence of radical polymerization initiators and optionally in the presence of chain transfer agents; cf. EP-B 672 212, page 4, lines 13-37 or EP-A 438 744, page 2, line 26 to page 8, line 18.

The water-soluble synthetic polymer used is preferably an anionic copolymer obtainable by copolymerizing

-   (a1) N-vinylformamide, -   (b1) acrylic acid, methacrylic acid and/or alkali metal or ammonium     salts thereof, and -   (c1) optionally one or more monoethylenically unsaturated monomers     which are different from the monomers of groups (a) and (b).

The water-soluble synthetic polymer comprises preferably

-   (a) 10 to 95 mol % of units of the formula I, -   (b) 5 to 90 mol % of units of a monoethylenically unsaturated     carboxylic acid having 3 to 8 C atoms in the molecule, and/or the     alkali metal, alkaline earth metal or ammonium salts thereof, and -   (c) 0 to 30 mol % of units of at least one monoethylenically     unsaturated monomer which is different from the monomers of     groups (a) and (b).

Particular preference is given to copolymers and terpolymers containing acrylic acid and vinylformamide in copolymerized form.

The water-soluble synthetic polymer used is preferably a water-soluble synthetic polymer obtainable by copolymerizing

-   (a2) 50 to 90 mol % of N-vinylformamide, -   (b2) 10 to 50 mol % of acrylic acid, methacrylic acid and/or the     alkali metal or ammonium salts thereof, and -   (c2) 0 to 30 mol % of at least one monoethylenically unsaturated     monomer which is different from the monomers of groups (a) and (b).

Likewise preferably employed as water-soluble synthetic polymer is an anionic copolymer obtainable by copolymerizing

-   (a1) vinyl acetate and -   (b) one or more monoethylenically unsaturated monomers containing     acid groups, and/or the alkali metal, alkaline earth metal or     ammonium salts thereof, and -   (c) optionally one or more monoethylenically unsaturated monomers     which are different from the monomers of groups (a) and (b),

followed by partial or complete hydrolysis of the vinyl acetate units copolymerized into the polymer.

Likewise preferred as water-soluble synthetic polymers are copolymers obtainable by copolymerizing a monomer mixture comprising, preferably consisting of,

-   (a1) acrylamide, -   (b1) acrylic acid, methacrylic acid and/or the alkali metal or     ammonium salts thereof, and -   (c1) optionally one or more monoethylenically unsaturated monomers     which are different to the monomers of groups (a) and (b).

The water-soluble synthetic polymer comprises for example

-   (a) 10 to 95 mol % of units of acrylamide, -   (b) 5 to 90 mol % of units of a monoethylenically unsaturated     carboxylic acid having 3 to 8 C atoms in the molecule and/or the     alkali metal, alkaline earth metal or ammonium salts thereof, and -   (c) 0 to 30 mol % of units of at least one monoethylenically     unsaturated monomer which is different from the monomers of     groups (a) and (b).

Employed with particular preference as water-soluble synthetic polymer are copolymers of acrylamide with a compound selected from acrylic acid, methacrylic acid and the alkali metal or ammonium salts thereof, preferably of acrylamide with acrylic acid.

These water-soluble synthetic polymers contain in copolymerized form generally at least 30 wt %, in a preferred form at least 40 wt %, and in a very preferred form at least 50 wt %, and also, in general, not more than 90 wt %, preferably not more than 85 wt %, and in a more preferred form not more than 80 wt % of acrylamide, based on the total weight of the monomers.

These water-soluble synthetic polymers contain in copolymerized form generally at least 70 wt %, in a preferred form at least 60 wt %, and in a very preferred form at least 40 wt %, and also, in general, not more than 10 wt %, preferably not more than 15 wt %, and, in a particularly preferred form, not more than 20 wt % of a compound selected from acrylic acid, methacrylic acid, and the alkali metal or ammonium salts thereof, preferably acrylic acid, based on the total weight of the monomers.

According to another preferred embodiment, polyacrylic acids are also contemplated as water-soluble synthetic polymers.

Particularly suitable are water-soluble polyacrylic acids which have a low molecular weight. Low molecular weight pertains to an average molecular weight (M_(w)) of less than 50 000, preferably less than 20 000, and most preferably less than 10 000, e.g., less than 5000. Such polyacrylic acids may contain as comonomers, in copolymerized form, other monocarboxylic acids, dicarboxylic acid monomers, and their anhydrides, and also monoethylenically unsaturated monomers which are not carboxylic acids.

Examples of monocarboxylic acids are methacrylic acid, vinylacetic acid (3-butenoic acid), and acryloyloxypropionic acid. Examples of suitable dicarboxylic acid monomers are maleic acid, itaconic acid, mesaconic acid, fumaric acid, and citraconic acid. Also suitable for use are the anhydrides of the carboxylic acids, such as maleic anhydride.

Monoethylenically unsaturated monomers which are not carboxylic acids may be present in amounts in which they are soluble in the reaction mixture and the polymer produced is soluble in water. In each case the carboxyl-free monomer is less than 80% and preferably less than 50 wt % of the overall weight of all the monomers employed. Examples of suitable monoethylenically unsaturated monomers which are not carboxylic acids are alkyl esters of acrylic or methacrylic acid, such as methyl, ethyl, or butyl acrylate or methyl, butyl, or isobutyl methacrylate; hydroxyalkyl esters of acrylic or methacrylic acids, such as hydroxyethyl or hydroxypropyl acrylate or methacrylate; acrylamide, methacrylamide, phosphoethyl methacrylate, allyl or methallyl alcohols, esters, and ethers; acrylonitrile, vinyl acetate, styrene, vinylsulfonic acid or salts thereof, allylsulfonic acid and 2-acrylamido-2-methylpropanesulfonic acid or salts thereof.

Preferred polyacrylic acids are those which are acrylic acid polymers or acrylic acid copolymers containing in copolymerized form up to 30 wt %, based on all ethylenically unsaturated monomers, of ethylenically unsaturated comonomers selected from the group consisting of methacrylic acid, maleic acid (or anhydride), vinylsulfonic acid, allylsulfonic acid, and 2-acrylamido-2-methylpropanesulfonic acid.

The polyacrylic acid is preferably acrylic acid homopolymer.

A process for preparing such polyacrylic acids is described in EP 405 818 and also in WO 2012/104325, hereby incorporated by reference.

According to these processes, chain transfer agents, for regulating molecular weight, are added during the radical polymerization of acrylic acid. Preferred chain transfer agents are hypophosphorous acid or a salt thereof, such as a sodium hypophosphite monohydrate. Suitable accordingly are polyacrylic acids obtainable by polymerizing monoethylenically unsaturated monocarboxylic acids and optionally further monomers, using sodium persulfate as initiator, in the presence of a hypophosphite as chain transfer agent, with an alkaline neutralizing agent being present during the polymerization in an amount sufficient to neutralize at least 20% of the acidic groups.

Examples of suitable polyacrylic acids are those obtainable by polymerizing acrylic acid in a feed regime with peroxodisulfate as initiator in the presence of hypophosphite as chain transfer agent and in water as solvent, with water and optionally one or more ethylenically unsaturated comonomers being included in an initial charge, and with acrylic acid in an acidic, non-neutralized form, optionally one or more ethylenically unsaturated comonomers, an aqueous peroxodisulfate solution, and an aqueous hypophosphite solution being added continuously, and a base being added to the aqueous solution after the end of the acrylic acid feed, with the comonomer content not exceeding 30 wt %, based on the total monomer content.

Also contemplated as water-soluble synthetic polymers are amphoteric copolymers with an anionic overall charge, obtainable by copolymerizing

-   (a) at least one monomer selected from acrylamide and an     N-vinylcarboxamide of the formula

-   -   in which R¹, R²═H or C₁ to C₆ alkyl,

-   (b) at least one monoethylenically unsaturated carboxylic acid     having 3 to 8 C atoms in the molecule and/or the alkali metal,     alkaline earth metal or ammonium salts thereof, and optionally

-   (c) other monoethylenically unsaturated monomers, which are     different from the monomers (a) and (b), and optionally

-   (d) compounds which have at least two ethylenically unsaturated     double bonds in the molecule,

followed by partial elimination of groups —CO—R¹ from the formula I monomers copolymerized into the copolymer, with formation of amino groups, the amount of amino groups in the copolymer being at least 5 mol % below the amount of copolymerized acid groups of the monomers (b). In the case of the hydrolysis of N-vinylcarboxamide polymers, a secondary reaction forms amidine units, by reaction of vinylamine units with an adjacent vinylformamide unit. Hereinafter, the reporting of vinylamine units in the amphoteric copolymers always means the sum total of vinylamine units and amidine units.

The amphoteric compounds obtainable accordingly contain for example

-   (a) 10 to 95 mol % of units of the formula I, -   (b) 5 to 90 mol % of units of a monoethylenically unsaturated     monomer containing acid groups, and/or the alkali metal, alkaline     earth metal or ammonium salts thereof, -   (c) 0 to 30 mol % of units of at least one monoethylenically     unsaturated monomer which is different from the monomers (a) and     (b), -   (d) 0 to 2 mol % of units of a compound containing at least two     ethylenically unsaturated double bonds, and -   (e) 0 to 42 mol % of vinylamine units, in copolymerized form, with     the amount of amino groups in the copolymer being at least 5 mol %     below the amount of copolymerized monomers (b) containing acid     groups.

The anionic copolymers may be hydrolyzed in the presence of acids or bases, or else enzymatically. In the case of hydrolysis with acids, the vinylamine groups formed from the vinylcarboxamide units are present in salt form. The hydrolysis of vinylcarboxamide copolymers is described at length in EP-A 438 744, page 8, line 20 to page 10, line 3. The statements made therein apply correspondingly to the preparation of the amphoteric polymers for inventive use.

The aqueous surface coating composition of the invention comprises, and preferably consists to an extent of at least 95 wt % of, consisting more particularly to an extent of 100% by weight of, the inventive composition of (A), (B), and (C) and water, with the solids content of the surface coating composition being 1 to 55 wt %, preferably 5 to 20 wt %, more particularly 10 to 15 wt %.

The aqueous surface coating composition of the invention preferably comprises

-   (A) 1 to 20 parts by weight, preferably 2 to 12 parts by weight,     more particularly 3 to 8 parts by weight of starch (solids), -   (B) 0.01 to 20 parts by weight, preferably 0.2 to 10 parts by     weight, more particularly 0.5 to 2 parts by weight of a zirconium     carbonate compound, -   (C) 0.01 to 40 parts by weight, preferably 0.5 to 20 parts by     weight, more particularly 2 to 10 parts by weight of water-soluble     synthetic polymer (solids), containing in copolymerized form one or     more monomers having monoethylenic double bonds.

The surface coating composition of the invention is applied to base paper. The amount of coating composition is applied with a coatweight of preferably 0.1 to 10 g/m².

The surface coating composition is produced by mixing of the individual components. According to one preferred variant, the boiled starch is introduced first and the water-soluble synthetic polymer is mixed in, followed by the zirconium carbonate compound.

The aqueous surface coating composition of the invention may further comprise a surface sizing agent based on an aqueous dispersion. The aqueous dispersions may possess anionic or cationic charge and they have a particle size of between 50 and 500 nm.

Examples of suitable dispersions are those obtainable by copolymerizing ethylenically unsaturated monomers, especially acrylonitrile and (meth)acrylates, and also, optionally, up to 10 wt % of further monomers such as styrene, by means of radically initiated emulsion polymerization in the presence of degraded starch. It is possible here for chain transfer agents to be used. Aqueous dispersions of this kind are described in EP 0 273 770, EP 0 257 412, WO 99/42490, WO 2002/14393, WO 2007/000419, WO 2007/000420, and WO 2011/039185, the disclosure content of which is expressly referenced.

The aqueous surface coating composition of the invention preferably has a viscosity in the range from 1 to 200 mPa·s (12% solids content and Brookfield spindle 2 at 100 rpm at which solids content (Brookfield LV viscosity, spindle 4, 6 rpm, RT).

The surface coating composition of the invention is suitable for coating base paper. The paper obtained accordingly is distinguished by high strengths.

The surface coating compositions of the invention may be processed by all of the methods suitable in a surface sizing context. Application with the surface coating composition of the invention takes place in a film and/or size press or by a contactless application technique with a spraying bar or curtain coating process. Coating may be accomplished by means of a doctor blade or a nozzle.

It is possible, generally speaking, to coat base paper in a separate film press and/or size press. The film press and/or size press is preferably arranged inline in the paper machine. Generally it is incubated into the drying unit. At the time of application, the paper sheet preferably has a water content of ≤60 wt %.

Application is made in the quantities customary in each case. For use, the surface coating composition is normally added to the size press liquor in an amount of 0.05 to 3 wt %, based on solids, guided by the desired degree of sizing of the papers to be furnished.

The present invention relates further to a method for producing paper and board, comprising the steps of

-   a) treating paper stock with paper auxiliary and/or filler, -   b) draining the paper stock treated by a), with sheet formation, and -   c) coating the paper web obtained by b) with the surface coating     composition of the invention, -   d) and drying the paper web coated by c).

Fibrous material used in accordance with the invention may comprise virgin and/or recovered fibers. Any softwood or hardwood fiber typically used in the paper industry may be employed, examples being unbleached chemical pulp, and also fibrous materials from any annual plants. Mechanical pulp includes, for example, groundwood, thermomechanical pulp (TMP), chemithermomechanical pulp (CTMP), pressure groundwood, semichemical pulp, high-yield pulp, and refiner mechanical pulp (RMP). Sulfate, sulfite, and soda chemical pulps are contemplated, for example. Suitable annual plants for producing fibrous materials include, for example, rice, wheat, sugarcane, and kenaf.

The pulp furnishes are preferably produced using waste paper, which is used either alone or in a mixture with other fibrous materials.

A preferred method for producing paper and board comprising the steps of

-   -   a) treating paper stock with paper auxiliary and/or filler,     -   b) draining the paper stock treated by a), with sheet formation,         and     -   c) coating the paper web obtained by b) with the surface coating         composition of the invention,     -   d) and drying the paper web coated by c),         with the proviso that no polyvinyl alcohol fibers are part of         the pulp.

In the case of waste paper it is possible to use a fibrous material having a freeness of 20 to 50 SR. In general a fibrous material with a freeness of about 40 SR is used, which is ground during the production of the pulp furnish. Preference is given to using fibrous material having a freeness of ≤40 SR.

The method of the invention serves preferably for producing filler-containing paper. The filler content of the paper is generally 1-20 wt %, preferably 5 to 20 wt %, more particularly 10-15 wt %, based on dry total paper stock or on paper stock. Filler here, as is usual in papermaking, means inorganic pigment.

As well as the filler, generally at a fibrous material concentration of 5 to 15 g/l, it is possible optionally for customary paper auxiliaries to be admixed to the paper stock. Examples of conventional paper auxiliaries are sizing agents, wet strength agents, cationic or anionic retention agents based on synthetic polymers, and also dual systems, dewatering agents, optical brighteners, defoamers, biocides, and paper dyes. These conventional paper additives may be used in the customary amounts.

Stock sizing agents include alkylketene dimers (AKDs), alkenylsuccinic anhydrides (ASAs), and rosin size.

Wet strength agents are synthetic dry strengtheners such as polyvinylamine, or natural dry strengtheners such as starch.

Retention aids suitable include, for example, anionic microparticles (colloidal silica, bentonite), anionic polyacrylamides, cationic polyacrylamides, cationic starch, cationic polyethyleneimine, or cationic polyvinylamine. Further conceivable are any desired combinations thereof, examples being dual systems consisting of a cationic polymer with an anionic microparticle or of an anionic polymer with a cationic microparticle. To achieve high filler retention, it is advisable to add such retention aids as may be added, for example, to thin stuff as well as to thick stuff.

Drainage takes place on the wire of the paper machine, with sheet formation. The paper web obtained accordingly then passes through the press section, in which the paper web is generally dried to a solids content of <40 wt %. This is followed by further dewatering by drying.

The coating step of the invention takes place during the drying phase. Depending on the paper machine, there may already be a drying unit upstream of the coating apparatus, preferably a size press.

The present specification relates further to the paper coated with the surface coating composition of the invention. Corrugated boards produced using this paper exhibit enhanced strength properties.

EXAMPLES

The examples which follow illustrate the present invention. The percentages in the examples are by weight, unless otherwise stated.

The K value of the polymers was determined according to Fikentscher, Cellulose-Chemie, Volume 13, 58-64 and 71-74 (1932) at a temperature of 20° C. In this connection, K=k·1000.

The solids content was ascertained by temperature-conditioning a sample of the product (approximately 3 g)—that is, drying it to constant weight—in a preheated forced air drying cabinet at 120° C.

Polymers used in the inventive and comparative examples were as follows.

Polymer (1) Vinylformamide/Acrylic Acid Copolymer (90/10 mol/mol)

In the initial charge, in 46.8 kg of water, the pH was adjusted to 6.6 using 0.35 kg of 85 wt % strength aqueous phosphoric acid and 0.65 kg of 25 wt % strength sodium hydroxide. 200 g of Afranil T (liquid) (defoamer) were added. The initial charge was then flushed with nitrogen and heated to 80° C., and a pressure of 450 mbar was established. The appearance of distillate was the cue for metered addition over 4 hours, from a feed, of 32.5 kg of vinylformamide (99% form), 14.8 kg of 32% strength aqueous sodium acrylate solution, and 8.9 kg of water. Concurrently, 7.4 kg of a 10 wt % strength aqueous solution of 2,2′-azobis(2-amidinopropane) hydrochloride were metered in over 4 hours.

After the end of the feeds, polymerization was continued for a further hour, after which a further 7.4 kg of 10 wt % strength aqueous 2,2′-azobis(2-amidinopropane) hydrochloride solution were metered in over a period of 15 minutes, followed by polymerization for 2 hours more. During the polymerization, 8.9 kg of water were distilled off. The product obtained accordingly may be characterized as follows:

K value 50.2 (determined in a 5 wt % strength aqueous sodium chloride solution at a pH of 7 and a polymer concentration of 1%)

Solids content (SC): 36 wt %

Polymer (2) Polyvinylformamide

With 62.7 kg of water, a pH of 65 was adjusted in the initial charge using 0.36 kg of 85 wt % strength aqueous phosphoric acid and 0.59 kg of 25 wt % strength sodium hydroxide. The initial charge was then flushed with nitrogen and heated to 80° C., and a pressure of 450 mbar was established. Appearance of distillate was the cue for metered addition over 3 hours of 38.0 kg of vinylformamide (99% form).

Concurrently, 0.83 kg of a 10 wt % strength aqueous solution of 2,2′-azobis(2-amidinopropane) hydrochloride were metered in from a feed over a period of 3 hours. 30 minutes after the start of the two feeds, the feed of 15.7 kg of water was commenced, over 3 hours 30 minutes. After the end of the monomer feed, the reaction mixture was held at 80° C. for 3 hours more. During the whole polymerization, 15.7 kg of water were distilled off. The product obtained accordingly may be characterized as follows:

K value: 42.9 (determined in water at a pH of 7 and a polymer concentration of 1%)

SC: 34.33%

Polymer (3) Hydrolyzed Copolymer of Vinyl Acetate/Crotonic Acid (90/10)

Included in the initial charge were 70 kg of water and 40 kg of 25 wt % strength sodium hydroxide. Dissolved with stirring over 60 minutes were, in portions, 21.6 kg of Luviset® CA 66 (from BASF, VAc/crotonic acid 90/10 mol/mol).

After the end of the addition, the solution was heated at an internal temperature of 80° C. and held at the temperature for a further 5 hours. The solution was then cooled and a pH of 6 was set by addition of 2.2 kg of 32% strength hydrochloric acid. The product obtained accordingly may be characterized as follows:

Degree of hydrolysis of VAc: 68%.

22.5% nonvolatile fraction (NVF); polymer content 18.0 wt %

Polymer (4) Copolymer of Acrylamide and Acrylic Acid (90:10 mol:mol)

In the initial charge, 80 kg of water, 1.38 kg of a dilute solution of Trilon® C (from BASF, diluted to 1 wt % solids content), and 375 g of sodium hypophosphite monohydrate (solid) were heated to 60° C., during which nitrogen was passed through the charge. 9.8 kg of 32 wt % strength aqueous sodium acrylate solution (33.34 mol) and 42.5 kg of 50 wt % strength aqueous acrylamide (301.07 mol) were metered in over a period of 2 hours 20 minutes. Concurrently, 12.6 kg of a 4 wt % strength aqueous solution of 2,2′-azobis(2-amidinopropane) hydrochloride were metered in over 2 hours 20 minutes. An hour after the end of the feeds, 2.76 kg of 4 wt % strength aqueous 2,2′-azobis(2-amidinopropane) hydrochloride solution were metered in over 5 minutes, after which the mixture was heated to an internal temperature of 80° C., thereby initiating the postpolymerization. The mixture was held at 80° C. for 2 hours. Then the pH was set to 6.3 by addition of 0.36 kg of 32% strength hydrochloric acid. The product obtained accordingly may be characterized as follows:

K value: 28.8 (determined in 5 wt % strength aqueous sodium chloride solution at a pH of 7 and a polymer concentration of 2%)

AM/AA ratio in the polymer: 90/10 mol/mol

SC: 17%

Polymer (5) Hydrolyzed Copolymer of Vinyl Acetate (VAc) and Vinylformamide (VFA)

In the initial charge, with 31.7 kg of water, a pH of 6.5 was set with 0.22 kg of 85 wt % strength phosphoric acid and 0.42 kg of 25 wt % strength sodium hydroxide. 2.33 kg of a 10% strength aqueous solution of Mowiol® 40-88 (from Kuraray) were added.

The initial charge was then flushed with nitrogen and heated to 65° C.

8.35 kg of vinylformamide (99%) were metered in over 3 hours. Commenced concurrently was the supply of 15.0 kg of vinyl acetate over 0.5 hour, and of 8.18 kg of a 2 wt % strength aqueous solution of 2,2′-azobis(2-amidinopropane) hydrochloride, over 8 hours.

After the end of the initiator feed, 5.0 kg of water were added and polymerization was continued for an hour. Then 10.5 kg of 2 wt % strength aqueous 2,2′-azobis(2-amidinopropane) hydrochloride solution were metered in over 15 minutes and, after the end of the feed, 5.0 kg of water were added. The batch was heated to an internal temperature of 70° C. and polymerization was continued for 2 hours, after which 25 kg of water were added. The pressure was lowered slightly, and 15 kg of water were distilled off. This gave a copolymer of vinyl acetate and VFA (47.6/52.4 mol:mol). The product mixture may be characterized as follows:

K value: 54.6 (determined in formamide with a polymer concentration of 1%)

SC: 22.8%

The product mixture obtained was then heated to an internal temperature of 80° C. Thereafter 0.6 kg of sodium bisulfate solution (40% in water) was added. When 80° C. had been reached, 3.2 kg of 25 wt % strength sodium hydroxide are added over 15 minutes. The mixture was hydrolyzed at this temperature for 3 hours and then cooled and adjusted to a pH of 8.2 using 8.7 kg of 32 wt % strength aqueous hydrochloric acid. This gave an end product with a polyvinyl alcohol content of 45.7%, a polyvinyl acetate content of 6.7%, a polyvinylformamide content of 27.8%, and a polyvinylformamide content of 19.8%. NVF: 22.4% WS: 15.8% polymer content: 9.8%

Polymer (6) Copolymer of Vinyl Acetate and VFA (50/50)

In the initial charge, with 31.7 kg of water, a pH of 6.5 was set with 0.22 kg of 85 wt % strength phosphoric acid and 0.42 kg of 25 wt % strength sodium hydroxide. 2.33 kg of a 10% strength aqueous solution of Mowiol 40-88 (from Kuraray) were added. The initial charge was then flushed with nitrogen and heated to 65° C. 8.35 kg of vinylformamide (99%) were metered in over 3 hours; concurrently, the supply of 15.0 kg of vinyl acetate commenced, over 0.5 hour, and of 8.18 kg of a 2 wt % strength aqueous solution of 2,2′-azobis(2-amidinopropane) hydrochloride, over 8 hours. After the end of the initiator feed, 5.0 kg of water were added and polymerization was continued for an hour. Then 10.5 kg of 2 wt % strength aqueous 2,2′-azobis(2-amidinopropane) hydrochloride solution were metered in over 15 minutes and, after the end of the feed, 5.0 kg of water were added. The batch was heated to an internal temperature of 70° C. and polymerization was continued for 2 hours, after which 25 kg of water were added. The pressure was lowered slightly and 15 kg of water were distilled off.

K value: 54.6 (determined in formamide with a polymer concentration of 1%).

SC: 22.8%

Ratio in the polymer: 47.6/52.4

Polymer (7)

Polymer 7 used was a polyacrylic acid having an average molecular weight (by GPC) of about 4000 g/mol and a degree of neutralization of 50.

Preparation of Starch Solution:

Merizet® 120 maize starch (from Tate & Lyle) was used, and was enzymatically degraded as follows: a 12% slurry of Merizet 120 was prepared in hot water at 65° C. under agitation in a 1000 L vessel, and 0.012% of PL 120 enzyme from Novozyme was added. After 20 minutes, 100 ml of acetic acid were metered into the starch solution to terminate the process of starch degradation. The starch solution had a viscosity of 55 mPas at 100 rpm (spindle 2).

Determination of the Mass Average (M_(w)) of the Starch Solution

The aqueous starch solutions were diluted with DMSO and thereby stabilized. The molar mass distribution was determined by GPC-MALLS (gel chromatography with multiangle laser light scattering). The GPC-MALLS consists of a Waters 515 pump module, devolatilizer, Waters 717 Autosampler, GPC column heating (Jet Stream). The MALLS detector is a Dawn-Heleos (Wyatt Technology, Santa Barbara, USA) equipped with a K5 flow cell and a He—Ne laser m from 10 to 658 nm and equipped with 16 detectors with an angle of 15 to 162° . The following GPC columns were used in series: Suprema S 30000, S 1000, and S 10 (PSS, Mainz, Germany). The samples were eluted with a DMSO-containing 0.09 M NaNO₃ solution with a flow rate of 0.5 ml/min and a temperature of 70° C. in the GPC columns. For software analysis, ASTRA 5.3.0.18 was used.

Determination of the Mass Average (M_(w)) of the Polymers

The molar mass distribution was determined by means of GPC-UV (gel chromatography with UV and fluorescence detector). The GPC consists of a Waters 515 pump module, devolatilizer, Waters 717 Autosampler, GPC column heating (Jet Stream). The UV detector is an Agilent (DRI 1200 UV) and the equipment also includes a fluorescence detector from Agilent (1200 VWD-260 nm). The following GPC columns were used in series: TSKgel GMPWXL. The samples were eluted with a 0.01 M NaN₃ solution with a flow rate of 0.8 ml/min and a temperature of 35° C. in the GPC columns. Prior to injection, the polymer solutions were filtered through a 0.2 micrometer Millipore filter. The concentration of the polymer solution was 1.5%.

Further compounds, used as auxiliaries:

Bacote 20® (from Zirconium Chemicals) is an alkaline solution of ammonium zirconium carbonate (Zirconate (2),bis[carbonato(2)-0]dihydroxydiammonium) with a solids content of 20%.

Production of Surface Coating Compositions

Using the polymers 1-7 and the above-prepared 12 wt % strength starch solution, surface coating compositions were produced. For this purpose, the starch solution was introduced first, and the polymer solution and Bacote 20 were metered in. The constitution of the surface coating composition was selected so as to achieve the amounts indicated in the table in parts by weight of starch (solids), parts by weight of polymer (solids), and parts by weight of Bacote 20. Each of the compositions was made up with water so as to give a solids content of 12 wt %. 2 parts by weight of Bacote 20 were used in each case, corresponding after conversion to 0.4 part by weight of ammonium zirconium carbonate.

Surface coating Starch composition [parts by weight] Polymer Bacote 20  1 (comparative) 12 — —  2 n.i. 8 4 pbw Polymer 1 —  3 8 4 pbw Polymer 1 2 pbw  4 n.i. 8 4 pbw Polymer 2 —  5 8 4 pbw Polymer 2 2 pbw  6 n.i. 8 4 pbw Polymer 3 —  7 8 4 pbw Polymer 3 2 pbw  8 n.i. 8 4 pbw Polymer 4  9 8 4 pbw Polymer 4 2 pbw 11 n.i. 8 4 pbw Polymer 5 12 8 4 pbw Polymer 5 2 pbw 13 n.i. 8 4 pbw Polymer 6 14 8 4 pbw Polymer 6 2 pbw 15 n.i. 8 4 pbw Polymer 7 16 8 4 pbw Polymer 7 2 pbw n.i.: not inventive pbw: parts by weight

Paper Coating

Since no large-scale pilot experiment was carried out, dry base paper was coated with the surface coating composition. Since the paper is subsequently dried again, the effect on the paper properties is negligible. The base paper used was composed 100% of waste paper (mixture of the following grades: 1.02, 1.04, 4.01) with a grammage of 100 g/m2, possessing no surface starch.

The base paper was coated in the formulations described in table 1 using a film press at 800 m/min on an experimental coater unit with IR dryers. The coatweights were determined gravimetrically. The coatweight reported is based on the dried amount of coating after departure from the IR dryer. The strength of the example papers was then investigated. Coating took place with different amounts of surface coating composition.

Performance Testing of the Base Papers

Prior to the testing of the paper, it was stored at 50% humidity for 24 hours, and the following strength investigations were conducted:

-   -   CMT according to DIN EN 23035 (Corona medium test)

The results of coating with a coatweight of 2 g/m² can be seen in table 2. This corresponds to an application quantity of 0.7 g/m² polymer (solids) and 1.3 g/m² starch (solids) with the addition in the inventive examples of Barcote 20.

TABLE 2 Performance results of the paper with the surface coating compositions of examples 2-16 at a coatweight of 2 g/m² Example CMT factor Increase¹⁾ Paper formulation Polymer [N · m2/g] [%]  1  1 — 1.97 6  2a  2 n.i. 1 2.16 16  3a  3 1 2.19 19  4a  4 n.i. 2 2.00 8  5a  5 2 2.09 13  6a  6 n.i. 3  7a  7 3  8a  8 n.i. 4 2.00 9  9a  9 4 2.09 14 11a 11 n.i. 5 2.04 11 12a 12 5 2.11 15 13a 13 n.i. 6 2.00 13 14a 14 6 2.10 16 15a 15 n.i. 7 2.01 10 16a 16 7 2.08 15 ¹⁾Increase in % by comparison with the uncoated base paper

The results of coating with a coatweight of 4 g/m² can be seen in table 3. This corresponds to an application quantity of 1.3 g/m² polymer (solids) and 2.7 g/m² starch (solids) with the addition in the inventive examples of Barcote 20.

TABLE 3 Performance results of the paper with the surface coating compositions of examples 2-16 at a coatweight of 4 g/m² Example CMT factor Increase¹⁾ Paper formulation Polymer [N · m2/g] [%]  1 comparative  1 — 2.20 19  2b  2 n.i. 1 2.25 22  3b  3 1 2.43 31  4b  4 n.i. 2 2.25 22  5b  5 2 2.50 35  6b  6 n.i. 3  7b  7 3  8b  8 n.i. 4 2.24 22  9b  9 4 2.29 25 11b 11 n.i. 5 2.23 21 12b 12 5 2.43 32 13b 13 n.i. 6 2.11 19 14b 14 6 2.22 21 15b 15 n.i. 7 2.23 21 16b 16 7 2.40 31 ¹⁾Increase in % by comparison with the uncoated base paper

The results of coating with a coatweight of 6 g/m² can be seen in table 4. This corresponds to an application quantity of 2 g/m² polymer (solids) and 4 g/m² starch (solids) with the addition in the inventive examples of Barcote 20.

TABLE 4 Performance results of the paper with the surface coating compositions of examples 2-16 at a coatweight of 6 g/m² Example GMT factor Increase¹⁾ Paper formulation Polymer [N . m²/g] [%]  1  1 — 2.36 28  2c  2 n.i. 1 2.34 27  3c  3 1 2.50 37  4c  4 n.i. 2 2.31 26  5c  5 2 2.57 40  6c  6 n.i. 3  7c  7 3  8c  8 n.i. 4 2.22 21  9c  9 4 2.27 24 11c 11 n.i. 5 2.23 21 12c 12 5 2.62 42 13c 13 n.i. 6 2.23 21 14c 14 6 2.26 27 15c 15 n.i. 7 2.23 21 16c 16 7 2.42 32 ¹⁾Increase in % by comparison with the uncoated base paper 

1. An aqueous surface coating composition for paper and board with a solids content of 1 to 55 wt %, the coating composition comprising: (A) 0 to 20 parts by weight of a starch; (B) 0.01 to 20 parts by weight of a zirconium carbonate compound; and (C) 0.01 to 40 parts by weight of a water-soluble synthetic polymer comprising in copolymerized form at least one monomer comprising at least one monoethylenic double bond.
 2. The aqueous surface coating composition according to claim 1, wherein the water-soluble synthetic polymer has an average molecular weight M_(w)≤1 million daltons.
 3. The aqueous surface coating composition according to claim 1, wherein the water-soluble synthetic polymer comprises at least one monomer in copolymerized form selected from the group consisting of an acrylamide, a vinyl alcohol, a vinyl acetate, and an N-vinylcarboxamide of the formula (I):

in which R¹, R²═H or C₁ to C₆ alkyl.
 4. The aqueous surface coating composition according to claim 1, wherein the water-soluble synthetic polymer is an anionic polymer.
 5. The aqueous surface coating composition according to claim 1, wherein the water-soluble synthetic polymer is obtainable by copolymerizing a monomer mixture comprising: (a) at least one monomer selected from the group consisting of an acrylamide and an N-vinylcarboxamide of the formula (I):

in which R¹, R²═H or C₁ to C₆ alkyl; (b) at least one monoethylenically unsaturated monomer comprising an acid group, an alkali metal salt of an acid group, an alkaline earth metal salt of an acid group, and an ammonium salt of an acid group, or a mixture thereof; (c) optionally at least one monoethylenically unsaturated monomer which is different from the monomers (a) and (b); and (d) optionally at least one compound having at least two ethylenically unsaturated double bonds in the molecule.
 6. The aqueous surface coating composition according to claim 1, wherein the water-soluble synthetic polymer is a copolymer or terpolymer comprising in copolymerized form monomers acrylic acid and vinylformamide.
 7. The aqueous surface coating composition according to claim 1, wherein the aqueous surface coating composition has a viscosity in the range from 1 to 200 mPa·s.
 8. The aqueous surface coating composition according to claim 1, comprising: (A) 1 to 20 parts by weight of the starch; (B) 0.01 to 20 parts by weight of the zirconium carbonate compound; and (C) 0.01 to 40 parts by weight of the water-soluble synthetic polymer.
 9. A method for producing paper and board, the method comprising: a) treating a paper stock with a paper auxiliary, a filler, or both, to obtain a treated paper stock; b) draining the treated paper stock with sheet formation, to obtain a paper web; and c) coating the paper web with the surface coating composition of claim 1, to obtain a coated paper web; and d) drying the coated paper web.
 10. A paper or board obtained by the method according to claim
 9. 11. A method for producing corrugated board, the method comprising corrugating the board of claim 10 to obtain a corrugated board.
 12. A corrugated board obtained by the method of claim
 11. 